Heat treatment of aluminous metals



- solutions. Because Patented Sept. 7, 1937 Philip T. Stroup,

New Kensington, to Aluminum Company of America,

Pa., assignor Pittsburgh,

Pa., a corporation of Pennsylvania No Drawing.

Application October 1, 1936,

Serial No. 103,534 12 Claims. (Cl. 118-131) This invention relates tothe thermal treatment of aluminum and aluminum base alloys. It is moreparticularly concerned with protecting such materials against a type ofattack which may occur at elevated temperatures under some atmosphericconditions.

Some kind of thermal treatment is generally used during the fabricationof articles from aluminum and aluminum base alloys, such as preheatingthe metal before hot working it, or applying a solution heat treatmentto certain alloys to increase their strength and hardness. Thesetreatments are frequently carried out in furnaces having air atmospheresbecause such furnaces are convenient and economical to operate. It hasbeen observed occasionally that when aluminum and aluminum base alloyshave been exposed to sufliciently elevated temperatures for aconsiderable length of time in an air atmosphere, the metal may besubject to a certain type of attack which differs from the surfaceoxidation that normally occurs with aluminum. The attack ischaracterized by such an objectionable blistering and permanentdiscoloration of the metal surface that the articles must usually bediscarded, since it is generally not economical to reclaim them.

The blisters which appear under the above described conditions arereadily distinguishable in shape and mode of occurrence from thoseblisters that are sometimes found on sheets of annealed metal. Thelatter type of blister results from the release of sorbed gas in themetal during the annealing operation, and for this reason it is commonlyreferred to as a gas blister. The blisters which characterize the hightemperature attack may even occur on metal that is substantially freefrom gas. The discoloration referred to hereinabove varies between adull gray and a black,'and it is further characterized by a fusedappearance, although there is no microscopic evidence of any incipientfusion. This discoloration is easily distinguishable from the dull oxidefilm that is normally found on aluminum and aluminum base alloys.Furthermore, it is of so permanent a nature that it is impractical toremove it by the ordinary commercial etching this type of attack differsfrom normal oxidation, and for the sake of convenience in referring toit, I shall designate it hereinbelow as high temperature blistering.

An investigation of the instances where high temperature blistering hasoccurred disclosed the fact that there is a greater susceptibility toblistering where the air atmosphere contains certain constituents suchas water vapor, ammonia, or sulphur compounds. It has also been foundthat the presence of certain elements in the alloys, notably magnesiumin combination with copper, nickel, silicon and/or zinc, tends toincrease the susceptibility to this attack. In practically all cases,the blistering occurred at temperatures above about 800 R, which isabove the range usually employed for annealing the cold worked metal. 1

Although aluminum and aluminum base alloys do not always suffer fromhigh temperature blistering, even at temperatures above 800 R, suchblistering does sometimes occur, and because of its objectionablecharacter, it becomes important to minimize or eliminate even anoccasional occurrence. A principal object of my invention is to providea method of inhibiting this high temperature blistering withoutimpairing any of the useful properties of the alloys subject thereto. Afurther object is to provide a method of protecting aluminum andaluminum base alloys which does not require the installation of newequipment and is adapted to use in the ordinary type of air furnace.

I have discovered that the foregoing objects may be realized byintroducing intoa furnace employed for thermal treatment certainfluorine compounds which are capable of producing a vaporous product,and allowing them to volatilize or decompose under the influence ofheat. as the case mav be. The presence of these fluors inc-containingproducts in the air at elevated temperatures inhibits the hightemperature blistering of solid aluminum and alumi rnim base alloys,without adversely affecting the useful properties of the metal orinterfering with any subsequent finishing operations. Only a relativelysmall amount of the fluorine-bearing substances, from a determinabletrace up to about 0.1 pound per cubic foot of heating chamber volume, isneeded to accomplish this result. The protection afforded by thesesubstances extends over the entire temperature range in which the attackoccurs, and it is particularly effective in the range of maximumsusceptibility between 800 F. and the temperature of incipient fusion.Aluminum base alloys are more susceptible to high temperature blisteringthan aluminum, and of the aluminum base alloys, those containing betweenabout 0.1 and 12 per cent magnesium apparently have a greater tendencyto blister. Other elements such as copper, silicon and zinc may also bepresent in the alloy in amounts of from 0.1 to 14 per cent, and theseelements, especially in combination with magnesium, appear to render thealloys even more susceptible to high temperature blistering. Aluminumbase alloys containing up to 5 per cent nickel, as well as such elementsas manganese, chromium, titanium, molybdenum, tungsten and the like inamounts of less than 2 per cent, may also be subject to this the majorcomponent of the alloy aside from aluminum, or as a minor constituent,as in the example given hereinbelow.

The high temperature blistering of aluminum alloys becomes importantonly where temperatures above about 800 F. are encountered. Thedeleterious attack does not occur at the usual annealing temperatures of650 to 700 F., but may occur at temperatures in excess of 800 F. such asare included in the temperature ranges normally employed for preheatingingots, billets, slabs, etc., before hot working them, or for solutionheat treatment of aluminum base alloys. The temperatures used in suchthermal treatments seldom, if ever, exceed 1100 F., and therefore highertemperatures may be disregarded. However, my method of protecting thealloys herein described is eflactive even at temperatures above 1100 F.

A protective atmosphere containing the vaporous fluorine-bearingcompounds makes it unfound in the air, namely, water vapor, ammonia, andsulphur compounds. Since it is both expensive and time-consuming toremove these subsuch as sulphur compounds, the term atmosphere or air asherein employed includes the presence of these constituents.

The fluorine constituent operation,

or by decomposition caused by reaction with substances such as watervapor, carbon dioxide, or sulphuric acid. The important feature is thatsome vaporous fluorine-bearing substance be produced.

below 1000 F., are suitable. pounds there are included sodiumfluosilicate, 895mm fluoborate, potassium 'sium fluobofat', aluminumfluoride, zinc fluoride, ammonium fluoborate, ammonium fluoride. Thegas, hydrogen fluoride, also inhibits this kind of dichloro-methane,dichloro-tetrafluoro-ethane, and trifluoro-monochloro-methane, are alsouseequivalent, fluorine-containing substances. My

invention comprehends the use of any of these substances, separately orin combination.

The amount of fluorine-containing substance furnace temperaturedecreases, 800 F., as compared with 950 or 1000 F.

When starting a furnace aooaoaa 0.064 inch in thickness was heated for20 hours at 920 F. in an atmosphere saturated with water vapor. The20-hour periodis muchlonger than is ordinarily employedin commercialpractice, butit was employed in order to obtain a severe attack and todetermine theeifectiveness of the inhibitor under such conditions; Othersamples of sheet from the same alloy were divided intothree' groupsandheat treated at the same temperature for the same length or time, v20hours, in water] vaporesaturated atmospheres containing afluorine-bearing eonstituentderived fromgheating three differentsalts'ynamelydsodium fluoborate, sodiumfluosilicate and [zinc flui5oride. About 0.005 pound of 'salt per cubic foot of furnace volume'was'placedon the floor of the furnace at the beginning of the heat andallowed to decompose or volatilize. No other additions of thesalt weremade during the 20-hour 20 period. The specimens that had been heated inthe non-fluoride-containing atmosphere were badly blistered anddiscolored, whereas those that had been exposed to the fluoride-bearingatmosphere were entirely free from blisters and the dark color. Thefluoride products obtained from heating the diiierent fluoride saltswere equally effective in inhibiting the attack, there being nosignificant difference in the appearance of the specimens in the threegroups tested. 80 An even more severe test was made on the protectiveeffect of a fluorine-containing substance by heating sheets of theforegoing alloy in a steam atmosphere which, of course, is 100 per centwater vapor. One group'of samples was heated for hours at 920 F., andthe second group was heated for the same length of time and at the sametemperature in a steam atmosphere containing fluorine substances derivedfrom the heating of 0.005 pound of aluminum fluoride a) per cubic footof heating chamber volume. Again, the specimens which were heated in thefluorine-bearing atmosphere were free from blisters and discoloration,while the other specimens were severely blistered and were nearly blackin color. Similar results were obtained when heating for shorterperiods.

Articles made from aluminum and aluminum base alloys and heated inatmospheres containing vaporous fluorides, as described hereinabove,

may be worked and formed or given any finish ing treatment to which sucharticles are normally subjected, without requiring any specialpreparations or extra operations by reason of having been exposed to theaforesaid atmospheres.

The term aluminum as herein employed refers to the metal of commercialpurity containing the usual impurities, and the term aluminum basealloys refers to those alloys containing more than 50 percent aluminum.Both aluminum and aluminum base alloys are comprehended within the termaluminous metal or material.

Having thus described my invention and certain embodiments thereof, Iclaim:

1. The method of inhibiting the high temperature blistering of aluminousmetal during thermal treatment, comprising heating a fluorinecontainingcompound capable of yielding a vaporous fluorine-containing productunder the influence of heat at temperatures below 1000 F.,

, and supplying said vaporous product to the atmosphere where saidthermal treatment is conducted.

2. In the art of thermally treating aluminous metals, the method ofinhibiting the high temperature blistering of said aluminous metal whenheated in an air atmosphere, comprising introducing into the furnaceatmosphere where the metal is heated, between 0.0001 and 0.1 pound oi avaporizable fluorine-containing compoundv per cubicfoot of heatingchamber volume, and heatingsaid metal insaid atmosphere.

3. In the art of thermally treating aluminous metals, the method ofinhibiting high temperature blistering of said aluminous metal duringheatingin an air atmosphere above about 800 F. but below the point ofincipient fusion, comprising employing a vaporous fluorine-containingcompound in the atmosphere surrounding the heated metal, saidfluorine-containing substance being derived from heating afluorine-containing compound capable of yielding avaporous product attemperatures below1000 F.

4. Inthe art of thermally treating aluminum base alloys containing from0.1 to 12 per cent magnesium, the method of inhibiting the hightemperature blistering of said alloys when they are heated in a gaseousatmosphere, which comprises providing in said atmosphere as vaporousfluorine-containing substance derived from heating a fluorine-containingcompound capable of yielding a vaporous product at temperatures below1000" F.

5. The method of inhibiting the high temperature blistering of aluminumbase alloys which contain from 0.1 to 12 per cent magnesium when theyare subjected to thermal treatment in an air atmosphere, comprisingemploying in the said atmosphere a vaporous fluorine-containingsubstance derived from the heating of at least one compound selectedfrom the group composed of sodium fluosilicate, sodium fluoborate, po-

tassium fluosilicate, potassium fluoborate, zinc fluoride, aluminumfluoride, ammonium fluoride, ammonium fluoborate, hydrogen fluoride,boron fluoride and difluoro-dichloro-methane;

6. The method of inhibiting the high temperature blistering of aluminumbase alloys which contain from 0.1 to 12 per cent magnesium duringthermal treatment in an air atmosphere at temperatures above about 800F. but below the point of incipient fusion, comprising employing avaporous fluorine-containing substance in the atmosphere surrounding themetal during said treatment, said fluorine-containing substance beingderived from heating a fluorine-containing compound capable of yieldinga vaporous product below 1000 F. under the influence of heat.

'7. The method of inhibiting high temperature blistering of aluminumbase alloys which contain 0.1 to 12 per cent magnesium when they aresubjected to thermal treatment in a furnace atmosphere at temperaturesbetween 800 F. and the temperature of incipient fusion, comprisingintroducing into the furnace atmosphere between 0.0001 and 0.1 pound ofa vaporizable fluorine-containing compound per cubic foot of heatingchamber volume.

8. In the art of thermally treating aluminum base alloys, the method ofinhibiting the high temperature blistering of said alloys when they areheated in an air atmosphere, comprising thermally treating the alloy inan air atmosphere containing a vaporous fluorine-bearing substancederived from heating solid sodium fluosilicate.

9. In the art of thermally treating aluminum base alloys, the method ofinhibiting the high temperature blistering of said alloys when they areheated in an air atmosphere, comprising thermally treating the alloy inan air atmosphere containing a vaporous fluorine-bearing substancederived from heating solid sodium fiuoborate.

10. In the art of thermally treating aluminum base alloys, the method ofinhibiting the high temperature blistering of said alloys when they areheated in an air atmosphere, comprising thermally treating the alloy inan air atmosphere containing a vaporous fluorine-bearing substancederived from heating solid aluminum fluoride.

11. In the art of thermally treating aluminum base alloys, the method ofinhibiting the high temperature blistering of an aluminum base alloywhich contains about 4.4 per cent copper, 0.5 per cent manganese and 1.5per cent magnesium when said alloy is heated in a furnace atmosphere totemperatures above about 800 F., comprising conducting the thermaltreatment in a furnace atmosphere containing a vaporous fluorine-bearingsubstance in an amount between 0.0001 and 0.1 pound per cubic foot orheating chamber volume.

12. The method of inhibiting the high temperature blistering oialuminous metals when subjected to thermal treatment at temperaturesabove about 800 F., comprising supplying to the furnace atmosphere wheresaid treatment is conducted a vaporous fluorine-bearing substance in anamount corresponding to at least about 0.0001 pound per cubic foot ofatmosphere, and conducting the treatment in said atmosphere.

